1) Field of the Invention
The invention relates to a process for the polymer modification of mineral coating agent and binder compositions by means of powders which are redispersible in water.
2) Background Art
The modification of powder paints, hydraulically setting systems, such as cementitious systems, gypsum-containing compositions and other pulverulent building materials by polymers which are redispersible in water is described in the prior art. The polymeric substances added usually act as binders. They often increase the elasticity and reduce the brittleness of the materials.
EP-A 149098 (U.S. Pat. No. 4,859,751) discloses, for example, redispersible powders based on vinyl chloride/ethylene copolymers as additives in hydraulic compositions, paints and coating compositions, adhesives and plastic plasters, to improve their mechanical properties. EP-A 407889 describes the use of redispersible polymer powders as additives for hydraulic binders to improve the mechanical strength. DE-A 4317036 describes polymer powders based on methyl methacrylate as additives in mineral binding building materials.
It is furthermore known from the prior art to employ organosilicon compounds, such as polysiloxanes or organosilanes, in powder form for hydrophobizing powder paints, hydraulically setting building materials or other pulverulent building materials. EP-A 228657 describes redispersible organosilicon compounds as hydrophobizing additives to plasters, hydraulic binders or paints. EP-A 508155 describes polysiloxane powders as hydrophobizing agents. EP-A 279373 discloses redispersible organopolysiloxane powders as additives in building materials.
Although these products are suitable as hydrophobizing agents, inter alia, for hydraulically setting compositions, their binder character is in general less pronounced than that of organic polymers. In addition, the use of organosilicon compounds as binders for application in building materials comprising mineral coating agent and binder compositions is often prohibited in practice for economic reasons and/or owing to considerations of performance, since if excessive amounts are added the processing properties are adversely affected.
In certain uses for mineral coating agent and binder compositions, a hydrophobizing effect is now also required, in addition to improved mechanical properties. It is known from the prior art to employ for this purpose dispersible powders which are obtained by spray drying an aqueous mixture of vinyl polymer or acrylic polymer and silicone (EP-A 493168) or are obtained by spray drying an aqueous mixture of organic polymer and organosilicon compound (DE-A 4402409).
Depending on the amount and types of organosilicon compounds(s) added to the aqueous mixture to be dried, binders which are hydrophobizing to a greater or lesser degree can thus be prepared. It is a significant disadvantage here that the requirements placed on such additives differ greatly, and for this reason a specific redispersible powder with a specific organic polymer/organosilicone mixing ratio must be prepared for each requirement, which results in a large number of different types of product. Co-spraying organic polymer dispersions with organosilicone emulsions which are incompatible with them, for example because of incompatible emulsifier/protective colloid systems, is also problematic.
The object was therefore to provide a process for modifying mineral coating agent and binder compositions with organic polymers and organosilicon compounds which ensures a higher flexibility in respect of the ratios of amounts of organic polymer and organosilicon content, compared with the prior art, and avoids the abovementioned incompatibilities.